Tank gauge system and level responsive device therefor



W. JOHNSON April 19, 1966 TANK GAUGE SYSTEM AND LEVEL RESPONSIVE DEVICETHEREFOR Filed Dec. 15, 1961 3 Sheets-Sheet 1 4 4a zj- April 19, 1966 w.JOHNSON 3,246,518

TANK GAUGE SYSTEM AND LEVEL RESPONSIVE DEVICE THEREFOR Filed Dec. l5.1961 5 Sheets-Sheet 2 y 40 @Www W. JOHNSON April 19, 1966 TANK GAUGESYSTEM AND LEVEL RESPONSIVE DEVICE THEREFOR 3 Sheets-Sheet 5 Filed DeC.13, 1961 United States Patent Oiice 3,246,518 Patented Apr. '19, "19663,246,518 TANK GAUGE SYSTEM AND LEVEL RESPONSIVE DEVICE THEREFOR WallaceJohnson, Macomb, Ill., assigner to Liquid Levels LectronicsIncorporated, a corporation of Illinois Filed Dec. 13, 1961, Ser. No.158,998 17 Claims. (Cl. 73 313) This invention relates to a tank gaugesystem and a level responsive device therefor.

The embodiment of the invention shown herein is particularly -welladapted for use in pressure vessels, such yas those used for storingpropane and butane, but my invention is also suitable for use withnon-pressurized `storage vessels such `as those used for naphtha, carbondioxide, corn products, anhydrous ammonia, refrigerants, and otherliquids having a high dielectric strength.

It is an object of my invention to provide an improved level indicatingsys-tem which can be used in -a tank` farm so that the level of any 1oneof a number of tanks can be measured from a central location Since sometank farms or storage facilities have as many as a hundred tanks, itbecomes important to provide a system wihch is relatively simple bothelectrically and mechanically.

The level responsive device in itself is of a simplified constrictionwhich permits a number of such devices to be electrically connected witha common electrical indicating means which also `includes switchingmechanism so that the level in any one of the devices can be checkedinstantaneously by actuating the switch mechanism.

One of the problems encountered in a level responsive device of thistype, which forms a part of an electrical circuit, is that of avoidingany sparking which would Icreate an explosion hazard.

According to my invention I have provided a level indicating devicewhich includes an electric circuit which can .be energized by a sourceof potential of low voltage land low electrical capacity, as the term isused in the battery art. AMore particularly, my system can be powered bytwo flashlight batteries, and the device is oper- `attive even thoughthere may be one or two miles of wire connecting the level responsivedevice with the indicating mechanism.

Furthermore, the system may be arranged so that the mechanical elementsof the level responsive device are a-t ground potential, thuseliminating the use of insulating materials in the mounting means. Thisavoids the problem of sparking due to failure of insul-ation.

It is afurther object of my invention to provide, for the `tank end ofsuch a system, an improved level respon sive device which is of asimplified mechanical construction. More particularly, the levelresponsive device is .a float actuated rheostat which contains no movingparts other than the float, which requires no flexible electricalconnection between the iloat and the external circuit elements, andwhich embodies means which are effective to make good electrical contactwithout the use of special mechanism which has to be actuated for eachreading in order to urge the contact member into engagement with theresist-ance element.

Another object of my invention is to provide an improved sliding contactfloat device which is free from cooking and binding so that it willoperate for many months without requiring inspection, adjustment, orrepair.

According to this aspect of my invention, the buoyant means of the floatdevice is disposed on opposite sides of a sliding sleeve member so as toprovide a balanced force which will minimize cooking, the sleeve memberbeing of substantial length soy as to avoid binding.

Still another object is to provide a float device which is operativeunder the extremely high pressures encountered in this art.

A further object of my invention is to provide a level responsive devicein which the coil supporting member also serves as a guide for thefloat, thus dispensing with a separate guide member for lthe oat.

Other objects, features, and advantages of my invention will becomeapparent as the description proceeds.

With reference now to the drawings in which like reference numeralsdesignate like parts:

FIG. 1 is a vertical sectional View through a pressure storage tank,showing `a preferred embodiment of my invention;

FIG. 2 is an enlarged axial section through the pressure seal;

FIG. 3 is a horizontal section taken along l-ine 3 3 of FIG. 2;

FIG. 4 is a horizontal section taken along the line 4 4 of FIG. 2;

FIG. 4a is an enlarged section similar to FIG. 4 but showing the mannerin which the parts are united to each other;

FIG. 5 is a vertical section showing the iloat device, taken along line5 5 of FIG. 1;

FIG. 6 is a plan view of the float device shown in FIG. 5;

FIG. 7 is a view, partly in section, showing the cooperation of thefloat device with the float guide;

FIG. 8 is a horizontal section taken along line 8 8 of FdG. 7;

FIG. 9 is an electrical diagram of the system;

FIG. 10 is an electrical diagram showing a modified form of theinvention;

FIG. 11 is a perspective View of a modified float device;

FIG. l2 is a horizontal section showing a modification of the floatguide; and

FIG. 13 shows a further modified float device.

With reference now to FIG. l, the reference numeral 10 indicates apressure tank for the storage of a liquid hydrocarbon. The tank isprovided with a manhole 11 and a manhole cover 12. A level responsivedevice 13 is disposed within the tank 10 and it is mounted in the topportion of the tank, such as the cover 12, by means of a pressure seal14and it extends down to the bottom of the tank. 'Ille level responsivedevice comprises a vertically disposed float guide 15 and a float device16 which cooperates therewith.

The support member, as shown in FIGS. 4 and 4a comprises a shell 20which is preferably in the form of an aluminum extrusion having ananodized surface, although it may be made out of an extrusion of asuitable resin material which is ystable with respect to the contents ofthe tank. The shell 20 is provided with lands 21 at each corner. Oneside of the shell is provided with a continuous slot 22 so that theshell is U-shaped. The opposite side of the shell is provided withundercut slots 23 (FIG. 4a) at a point adjacent the opposite wall 24.

A resistance element, preferably in the form of a coil of resistancewire 26, is received within the shell 20 so that a portion thereof is.exposed through the slot 22. Thus, the shell 20 serves both as a guidefor the float device 16 and a mounting means for the resist-ance wire26. In the preferred arrangement, the resistance Wire 26 is Wound on ahollow core element 25 which may also be formed of anodized aluminum.The resistance wire 26 is preferably provided with a coating ofinsulating enamel or other insulating material such as Formvar. At thepoint opposite the slot 22, the coating is removed, as by bufling.

In the arrangement shown, the parts are bonded together so that the core25 serves to reinforce the shell bly.

20 to provide a comparatively rigid structure. The

parts are of a relatively'loose fit to permit ease of asseml y As shownin FIG. 4a, the bonding is effected by a suitable bonding material suchas an epoxy resin 28. The wall 24 of the shell is provided with one ormore apertures 27, and after the core and shell have been assembled, butprior to the insertion of the hereinafter mentioned conductor strip 29,the resin 28 is injected into the space between the shell 20 and thecore 25 to secure the core with respect to the shell and also to provideperiodically spaced anchors for the convolutions of the resistance wire26.

After the parts have been thus assembled, a conductor strip 29 isinserted in the undercut slots 23 and overlies the ywall 24.

A modified arrangement is shown in FIG. 12 in which the core has beenomitted. Here the float guide comprises an extruded element 80 ofanodized aluminum and having an undercut recess 81 formed in one wallthereof, and a resistance coil 82 disposed in the recess 81. A conductorstrip 29 is mounted in the opposite surface. The convolutions of theresistance coil 82 may be held in place in the recess 81 by frictionalengagement (the coil having been maintained in a wound up condition atthe time of insertion), but preferably I provide a suitable cement orresin 83 to anchor convolutions to the bottom of the recess 81.

The resistance wire 26 may be formed of any standard nickel chromiumalloy suitable for this purpose, and the conductor strip 29 may beformed of the same material. For use with butane, propane, and the like,I have found Karma wire to be suitable, this being an alloy of 73%nickel, 20% chromium, and the remainder being aluminum and iron.

A lead 30 (FIG. 2) is brought through the wall of the core 25 andextended upwardly through the interior of the core and the pressure seal14.

Aluminum or aluminum alloy is a preferred material for the shell and thecore for the reason that it can be extruded, it is of light weight, andthe surface can readily be rendered both wear resistant andsubstantially nonconducting by anodizing.

The float device 16 includes contact means for making a connectionbetween the resistance wire 26 and the conductor strip 29 so as toregulate the amount of resistance in one or more arms of a bridgecircuit. The float device 16 includes a rectangular sleeve 31,preferably of aluminum, which is loosely mounted on the shell 2t) andmakes sliding contact with the lands 21. The inner surface ofthe sleeve31 carries oppositely disposed spring biased contact arms 32 and 33,preferably having gold contact points 58 and 59. Contact arm 32 engagesthe resistance wire 26 and contact arm 33 engages the conductor strip29. This arrangement is shown in FIGS. 5, 7 yand 8. Contact arms 32 and33 are electrically connected to each other by the material of thesleeve 31.

The float device 16 also includes four tubular float members v34, twobeing disposed on each side of the sleeve 31. The iloat members 34 aresecured to each other and urged inwardly intor contact with the sleeveby bands 35 and two screws 36. Epoxy welds 37 also secure the parts t-oeach other.

I have found that the ordinary type of hollow float is not suitable foruse in a butane pressure vessel because it is likely to collapse underthe extremely high pressures encountered, which may be of the order of300` pounds per square inch. Therefore, I have found that a veryeffective non-collapsible tioat means can be formed by using severaltubular float members which are of substantial length, but of a diameterwhich is sufficiently small with respect to the strength of the materialas to withstand the pressures involved. As shown in FIG. 6, each tioatmember 34 comprises a tubular section 38 having hemispherical end caps39 which are secured to each other rby epoxy welds 40.

Thus, a rigid and non-collapsible float structure is provided in whichthe ratio o=f .displafcement volume to sliding contact friction issufficiently great as to render the same sensitive to changes in theliquid level of the order of one-tenth of an inch, when installed in apropane tank. The large surface area and the distribution of areaprovides a balanced force acting on the sleeve 31 which, when coupledwith the substantial sleeve length, is effective to prevent binding andcockin-g of the float means 16.

FIG. l1 shows a modiiied type of float device 16', in which the floatmember 34 is formed of foamed metal, such as aluminum or magnesium, thismaterial being described in U.S. Patent No. 2,751,289, dated June 19,1956.

FIG. 13 shows a further modification, the oat device 16 being a ventedself hailing oat comprising a hollow float member 85. A vent tube 86extends through the upper wall thereof, the lower end terminating in asump 87. The vent tube permits the Iinterior of the float member to beat the same pressure as the interior of the tank 10. Should any liquidcondense within the float due to temperature or pressure changes itwould accumulate in the su-mp and consequently any slight temperatureo-r pressure change would creat\e a pressure differential sufcient toforce the liquid outf The ends of the coil of resistance wire 26 arebrought through the wall of the core 25 and extend upwardly through thecore, as shown in FIGS. 2 and 4 to provide leads 41 and 42 which extendout through the pressure seal 14, similar to the lead 30.

The upper end of the core 25 is received within the recess 46 of ahollow plug 44 and is secured in place by set screws 47. The plug 44 hasa small bore 48 for accommodating the leads 3), 41 and 42, and isfurther provided with a threaded counter bore for receiving a hollowpacking screw 49, and packing material 50. Thus the oat lguide 15 `atits upper end is mounted in and suspended from the plug 44. The plu-g 44is provided with screw threads 45 which cooperate with a similarlythreaded portion of a flanged bushing 51. The bushing 51 extends throughthe manhole cover 12 and is welded thereto at points 52. Surrounding theplug 44 at its upper portion is a screw threaded packing collar 53 whichcooperates with the bu-shing 51 and packing material 54 to provide apressure seal between the plug 44 and the bushing 51.

The system as a whole is shown in the electrical diagram of FIG. 9. Arheostat 60 is connected at each end across the leads 41 and 42. Therheostat has a contact arm 61, and a bridge connection 62 extends fromthe contact arm 61 to the sliding contact 32 through the lead 30 whichis common to each of the several level responsive devices sh-own. Thebridge connection 62, or the common lead 30 which forms a part thereof,is preferably grounded as at 63. A galvanometer 64 is interposed in thebridge connection 62. A low voltage, low electrical capacity battery 65is connected across leads 41 and 42. Thus a bridge circuit is providedof which the rheostat 60 forms the two arms of one branch, and theresistance coil 26 forms the two arms 0f the other branch. The slidingcontact 32 of the oat device 16 separates the two right hand arms, andthe contact arm 61 separates the two left hand arms.

Adjustment of the arm contact 61 in one direction or the other willcause the position of the needle of the galvanometer 64 to move in onedirection or the other. When the needle reads zero, then the position ofthe contact arm 61 indicates the level in the tank 10. Voltage limitingdiodes 66 may be connected across the galvanometer 64.

As shown in FIG. 9, the leads 41 and 42 may be connected by means ofganged switches 67 and 68'to any one of a plurality of level responsivedevices 13, 13a 13b, and 13C, located in as many different tanks. Thecommon portion of the circuit constitutes the control station 70 whichmay be placed at any convenient location. Adjustable resistances 71 maybe included in leads 41, 41a, 41b and 41C to compensate for thevariation in line length, and similar adjustable resistances 72 may beprovided with respect to the leads 42, 42a, 42h, and 42o. I have foundfor instance, that when the resistance of each branch of the bridgecircuit is in the neighborhood of 1000 to 1500 ohms that the cincuit maybe powered by two flashlight batteries.

In the bridge circuit of FIG. 9, the arrangement of the sliding contacts32, 61 with respect to coil 26 and rheostat 60 constitutes a voltagedivider connection. In the modification -of FIG. l0 the lower ends ofthe elements 26 Iand 60 are not connected to each other, although theformer may be connected to the common lead 30. Hence, the coil 26 andthe rheostat 60' constitute variable resistances each of whichconstitute only one arm of each branch of the bridge circuit. Therefore,lixed resistances 74 and 75 are connected into the circuit to providethe other arm of each branch, respectively. The advantage of the FIG.arrangement over the FIG 9 arrangement is that only a single separatelead (in addition to the common lead 3d) is required between each tankand the indicator device 70.

Indicia 76 are provided for the rheostat 60. Where the tanks areidentical, the indicia can read directly in gallons; otherwise aseparate set of indicia can be provided for each type of tank. In thealternative, the indicia 76 may react in percentage of total capacity.

The several level responsive devices 13 are calibrated with respect tothe indicia 76 by means of the adjustable resistances 71 and 72 so as toprovide accurate readings.

To summarize the operation, when it is desired to measure the contentsof tank 10, for instance, the ganged switches 67 and 68- are moved fromthe off position to a position registering with the proper leads. In theposition of the parts shown in FIG. 9, there will be a greater voltageat the contact arm 61 than at the sliding contact 32 with the resultthat the galvanometer 64 will indicate the flow of current. The contactarm 61 is therefore adjusted so that the galvanometer 64 reads zero. Atthis point, when the bridge circuit is balanced, the position of thecontact arm 61 is a function of the position of the i'loat device 16,with .the result that the liquid level will be indicated by the indicia76.

In order to checkthe level of the other tanks, the

switches 67 and 68 may be moved into contact with leads,

41o-42a and the operation repeated, and then into con tact with leads41b-42b, and so forth.

Preferably, the contact arms 32 and 33 engage the resistance wire 26 andthe conductor strip 29, respectively, with a comparatively lightpressure, so that the frictional force resisting displacement is of theorder of only a few ounces. For example, in the construction shown, acontact pressure of from four to eight ounces results in a frictionalresistance of two ounces. This contributes materially to the sensitivityof the device since only a very slight variation of liquid level isnecessary to create sufficient buoyancy, or the reverse, to slide thecontacts. Thus, it is desirable to make the contact points 58, 59 of asuitable material, such as gold, which will maintain good electricalcontact iu spite of the relatively light pressure being exerted.

As previously indicated, an outstanding advantage of my tank gaugesystem is that it eliminates the explosion hazard. There is no sparkingat contacts 32 and 33 for the reason that only a small amount of poweris involved. Two flashlight batteries provide three volts, and thecurrent running through the resistance wire 26 and the sliding contact32, 33 is only of the order of a few milli- ,amp'eres The leads 30, 41and 42 are enclosed in a grounded conduit, and the whole circuit iscompletely isolated from any commercial power line.

In the course of repair or calibration of the control station 70 it ispossible that a higher voltage could be inadvertently applied to somelead. This is an explosion hazard because the tanks are commonlygrounded as at 55 (FIG. 1). Thus, even though a non-sparking powersource is used, a hazard is presented of which, even though thepossibility of occurrence may be remote, the consequences would bedisastrous.

This hazard may be avoided by grounding the common lead 30 so that therewill be no potential difference between the conductor strip 29 and anyof the tanks 10.

The ground 63 is located at the control station end of the system, andpreferably between the galvanometer 64 and the sliding contacts 32.Additional grounds may be provided. Thus, the advantages of an isolatedsystem are obtained without `the necessity of insulating the conductorstrip 29 from the iioat guide 15, or the oat guide from the tank 10,either at the top (pressure seal 14) or at the bottom (point 56 of FIG.l).

Due to my improved arrangement, the float guide 15 can be permitted torest on the tank bottom at point 56 to reinforce the same againstvibration without requiring any insulation between the two, and noinsulation is required between the bushing 51 and the wall 12 of thetank 10. This feature simplifies cost of installation. The floats 16 areall at ground potential.

The system can be very easily installed; installation of a levelresponsive device in a tank involves only the making of an opening inthe tank cover 12, the welding in place of the bushing 51, and then theassembly of the screw threaded parts. The plug arrangement 44 permitscoarse adjustment of the length of the float guide 15 to be made by theset screws 47, and fine adjustment, in urging the bottom of the iloatguide into contact with the bottom of the tank, to be made by the screwthreaded means 45.

The level responsive device 13 has ben designed to eliminate mechanicaldefects which would require the removal of the cover 12 incident toinspection, adjustment, or repair, and consequent loss of propane vapor.More particularly, a simplicity of operation has been achieved bycombining the iioat guide and the coil holding structure as a singleelement, and the parts are arranged so that a balanced force is applied-to the float, thus avoiding cocking and binding.

Although my tank gauge system is suitable for use with any storage tankfor liquid chemicals of high dielectric strength, when used inconnection with propane and butane the level responsive device 13 mustbe able to withstand the very considerable pressures encountered. Forthis reason, a non-collapsible float construction has been used, ofwhich there are several suitable types as shown in FIGS. 5, 11, and 13.

While only preferred embodiments of my invention have been shown anddescribed herein, it will be understood that Various modifications andchanges may be made in the construction shown without departing from thespirit of my invention as pointed out in the appended claims.

'I claim:

1. A tank gauge system for a liquid chemical storage tank comprising, incombination,

a irst variable resistance device located in said tank and having atioat actuated sliding contact located in said tank,

a control station which includes a second variable resistance devicehaving a sliding contact,

means connecting said second variable resistance device to said iirstvariable resistance device,

means including a power source providing a bridge circuit which includessaid variable resistance devices,

said bridge circuit providing means including a bridge connectionbetween said two contacts,

and a null reading galvanometer interposed in said 'bridge connection,

said null reading galvanometer being located at said control stati-on,

said storage tank being grounded and said bridge connection beinggrounded, whereby no potential difference exists between said slidingcontact and said tank in which it is located.

2. A tank gauge system as claimed in claim 1 in which said connectingmeans includes a first lead means connecting `one end of said rstvariable resistance device with one end of said second variable'resistance device,

and a second lead means connecting the other end of said iirst variableresistance device with the other end `of the second variable resistancedevice,

whereby each of said variable resistance devices constitutes a voltagedivide-r.

3. A tank gauge system as claimed in claim 1 in which each of saidvariable resistance devices comprises one arm of each branch of saidbridge circuit, and

in which said ybridge circuit providing means includes two xedresistances, each constituting the other arm of each branch of saidbridge circuit,

the sliding contact of said second variable resistance device havingindicia associated therewith for indicating the relative position ofsaid float actuated sliding contact when said sliding contact of saidsecond variable resistance device is adjusted so that said galvanometerreads zero.

4. A tank gauge syste-m for a plurality of liquid storage tankscomprising, in combination,

a plurality of level responsive devices, each including a first voltagedivider hav-ing a oat actuated sliding contact,

each of said devices being disposed in one of said tanks and beingpartially submerged in the liquid contained therein,

a control station which includes a second voltage divider having anadjustable contact,

means selectively connecting said second voltage divider to one of saidfirst voltage dividers, and

means inclu-ding a power source providing a bridge circuit whichincludes said voltage dividers,

said bridge circuit including a bridge connection between said slidingcontact and said adjustable contact,

and a null reading galvanometer interposed in said bridge connection,

said null reading galvanometer being located at said control station,

said storage tanks being grounded, and said bridge connection includinga common lead electrically connected to all of said sliding contacts,said common lead -being grounded, whereby no potential difference existsbetween any sliding contact and the tank in which it is located.

5. A tank gauge system as claimed in claim 4 in which said 'bridgeconnection includes a common lead electrically connected to all of saidsliding contacts.

6. A tank gauge system as claimed in claim 5 in which said levelresponsive device includes a metallic float guide and said rst voltagedivider includes a coil of resistance wire supported by said iloatguide.

a oat device slidably mounted on said float guide,

said iirst sliding contact being mounted on said float device,

a second sliding contact mounted lon said float device and electricallyconnected to said first sliding contact for electing electricalconnection between said rst sliding contact and said common lead.

7. A level responsive device for a tank gauge system comprising avertically disposed iioat guide adapted for location in a liquid storagetank, and

a float device cooperating therewith,

said lioat guide having a vertically extending recess formed in asurface thereof,

a resistance coil mounted in said recess whereby a portion of saidresistance coil is exposed for cooperation with a sliding contact,

and a vertically extending low resistance/conductor strip mounted onanother surface of said oat guide, said iloa-t device comprising,

a sleeve surrounding said iloat guide,

float means connected to said sleeve,

a sliding contact mounted on said sleeve for cooperation with saidresistance coil, and

a second Contact electrically connected to said sliding Contact andmounted on said sleeve for cooperation with said conductor strip wherebysaid conductor strip provides a non-llex'ible low resistance electricconnection between said sliding contact and an electric circuit.

8. A level responsive device for a tank gauge system comprising avertically disposed float guide .adapted for location in a liquidstorage tank, and

a oat device cooperating therewith,

said float guide comprising,

a shell,

a core disposed within said shell,

a resistance coil mounted on said core,

a continuous slot in said shell whereby a portion of said resistancecoil is exposed for cooperation with a sliding contact, and

a low resistance return strip mounted on said shell,

said float device comprising a sleeve slidably mounted on said shell,

float means connected to said sleeve,

a sliding contact mounted on said sleeve for cooperation with saidresistance coil, and

a second contact mounted on said sleeve for cooperation with said returnstrip,

said contacts being electrically connected to each other.

9. A level responsive device as claimed in claim 3 in which said iioatmeans comprises a plurali-ty of sealed tubular iioat members mounted onopposite sides of said sleeve, said tubular float members being ofrelatively s-mall diameter so as to withstand a pressure ofsubstantially 300 pounds per square inch.

1Q. A level responsive device as claimed in claim 8 in which said ii-oat.means comprises a tloat member yformed of foamed metal.

11. A level responsive device as claimed in claim 8 in which said oatmeans comprises a hollow, vented, self-'bailing oat member.

12. A level responsive device as claimed in claim 8 in which said coreis bonded to said shell at spaced inter vals by an epoxy resin.

13. A level responsive device as claimed in claim 8 in which said shellis formed of anodized aluminum.

14. A level responsive device as claimed in claim 8 in which said shellis provided with lands for cooperation with said sleeve.

15. A level responsive device for a tank comprising,

a vertically disposed float guide located in said tank,

and including a shell,

a resistance coil mounted within said shell,

a continuous slot in said shell whereby a portion of said resistancecoil is exposed for cooperation with a sliding Contact,

and a low resistance return strip mounted on said shell,

a oat device slida'bly mounted on and cooperating with said oat guide,

two electrically connected sliding contacts mounted on said float deviceand engaging said resistance coil and said return strip, respectively,and

means for mounting said iioa-t guide in said tank.

16. A liquid level responsive device for a grounded pressure Itankcomprising a vertically disposed iioat guide located in said tank,

a iioat device slidably mounted on and cooperating with said floatguide,

said float guide including a resistance coil and a return strip, andsaid float device including sliding contact means cooperating kwith saidresistance coil and said return strip to provide a variable resistancedevice, pressure seal means for mounting said oat guide in the upperportion of said tank with the lower end of said iioat guide abutting thebottom of said tank,` said pressure seal mounting means including,

a threaded bushing mounted in the top wall of said tank,

an externally threaded hollow plug cooperating with said bushing inpressure tig-ht relationship,

said float guide being received at its upper end in said hollow plug,

set screws securing said iioat guide to said plug,

said return strip havi-ng a lead and said resistance coil having twoinsulated leads extending upwardly through said plug,

and meansfor sealing said three leads to said plug in pressure tightrelationship, said float guide being formed of a conducting material andbeing in electrical contact with said return strip,

whereby a grounding of said return strip lead will avoid the occurrenceof a potential difference across said set screws and the screw threadedconnection between said plug and said bus-hing.

17. A tank Vgauge system for a liquid storage tank which includes alevel responsive device including a first voltage divider located insaid tank, a control station including a second voltage divider havingan adjus-table contact 61, means includ-ing a power source and a bridgeconnection 30 providing a bridge circuit which includes said voltagedividers in each branch thereof, characterized in that said levelresponsive device includes a vertically disposed hollow oat guidedisposed in said tank with its lower end extending int-o the liquidcontents thereof, a iioat device cooperating therewith, said hollowlioat guide having a vertically extending slot formed in the surfacethereof, said first voltage divider including a resistance coilinsulated from and disposed within said hollow float guide and having aportion there-of exposed lby said slot, and a vertically extending lowresist-ance conductor strip mounted on the surface of `said oat guide,said conductor strip constituting one end of said bridge connection, theother end of said .bridge connection being connected to said adjustablecontact, and said float device including sliding contact means makingelectrical connection between said resistance coil and said conductorstrip, and a null reading galvanometer interposed in said bridgeconnection and located at said control station.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCESRoberts, H. C.: Mechanical Measurements by Electrical Methods,Pittsburgh, The Instruments Publishing Co. Inc., pages and 222. (Copy inGroup 430.)

LOUIS R. PRINCE, Primary Examiner.

ISAAC LISANN, Examiner.

S. H. BAZERMAN, Assistant Examiner.

17. A TANK GAUGE SYSTEM FOR A LIQUID STORAGE TANK WHICH INCLUDES A LEVELRESPONSIVE DEVICE INCLUDING A FIRST VOLTAGE DIVIDER LOCATED IN SAIDTANK, A CONTROL STATION INCLUDING A SECOND VOLTAGE DIVIDER HAVING ANADJUSTABLE CONTACT 61, MEANS INCLUDING POWER SOURCE AND A BRIDGECONNECTION 30 PROVIDING A BRIDGE CIRCUIT WHICH INCLUDES SAID VOLTAGEDIVIDERS IN EACH BRANCH THEREOF, CHARACTERIZED IN THAT SAID LEVELRESPONSIVE DEVICE INCLUDES A VERTICALLY DISPOSED HOLLOW FLOAT GUIDEDISPOSED IN SAID TANK WITH ITS LOWER END EXTENDING INTO THE LIQUIDCONTENTS THEREOF, A FLOAT DEVICE COOPERATING THEREWITH, SAID HOLLOWFLOAT GUIDE HAVING A VERTICALLY EXTENDING SLOT FORMED IN THE SURFACETHEREOF, SAID FIRST VOLTAGE DIVIDER INCLUDING A RESISTANCE COILINSULATED FROM AND DISPOSED WITHIN SAID HOLLOW FLOAT GUIDE AND HAVING APORTION THEREOF EXPOSED BY SAID SLOT, AND A VERTICALLY EXTENDING LOWRESISTANCE CONDUCTOR STRIP MOUNTED ON THE SURFACE OF SAID FLOAT GUIDE,SAID CONDUCTOR STRIP CONSTITUTING ONE END OF SAID BRIDGE CONNECTION, THEOTHER END OF SAID BRIDGE CONNECTION BEING CONNECTED TO SAID ADJUSTABLECONTACT, AND SAID FLOAT DEVICE INCLUDING SLIDING CONTACT MEANS MAKINGELECTRICAL CONNECTION BETWEEN SAID RESISTANCE COIL AND SAID CONDUCTORSTRIP, AND A NULL READING GALVANOMETER INTERPOSED IN SAID BRIDGECONNECTION AND LOCATED AT SAID CONTROL STATION.